In recent years, concerning the power consumption of thin display devices, low power consumption devices such as LED for a backlight (hereinafter, referred to as a B/L) used in liquid-crystal display devices have been developed. However, a value of B/L power consumption as related to total power consumption is still large when a display device or the like is used in the always on lighting condition.
In these display devices, a technique of B/L luminance controlled according to the input video signal has been known.
For example, when a wholly dark image has been inputted, it has been designed to reduce the B/L luminance and perform a gamma correction according to a reduced amount of luminance. As a result, low power consumption has been achieved by reducing the influence on visibility of a screen image. This method is called as CABC (Content Adaptive Brightness Control) control.
This method will be described in detail. In accordance with this method, when the input video signal is configured by the wholly dark gradation (low gradation), low power consumption can be achieved by increasing a reduction of B/L luminance, a gradation conversion volume (a rate of converting from low gradation to high gradation) and a transmittance rate of panel. When the inputted video signal is configured by the wholly bright gradation (high gradation), the original visibility of screen image can be obtained by the reduction of B/L luminance and the gradation conversion volume.
With this method, it is required to calculate a feature value of the video signal from the data (image data) in one frame of the inputted video signal in order to determine the reduction of luminance. In other words, as the reduction of luminance is determined by the feature value of the video signal, an image quality is affected by the feature value of the video signal. Thus, a calculation method of the feature value is important.
From a viewpoint of low power consumption, when the circuit itself for calculating the feature value gets too large, the power consumption increases in circuit. Thus, it is important to reduce a circuit scale.